Hansgeorg Pape

Core flooding laboratory experiment validates numerical simulation of induced permeability change in reservoir sandstone

[1]xa0Numerical simulation of reactive transport was validated in a core flooding experiment simulating conditions in a managed geothermal reservoir. Permeability was measured along a sandstone core prepared with anhydrite and subjected to a temperature gradient. Anhydrite was dissolved and precipitated in the cold upstream and hot downstream regions of the core, respectively. The numerical code SHEMAT was used to simulate coupled transport and chemical reactions at the temperature front (http://www.rwth-aachen.de/geop/shemat/). It comprises an extended version of the geochemical speciation code PHRQPITZ for calculating chemical reactions in brines of low-high ionic strength and temperatures of 0–150°C. Permeability is updated to porosity via a novel, calibrated power-law based on a fractal pore-space model resulting in a large exponent of 11.3. Simulation results agree well with measured permeability. This both validates the model and demonstrates that the fractal relationship is crucial for a successful simulation of this type of reactive transport.

Mobile NMR for porosity analysis of drill core sections

We apply a novel mobile nuclear magnetic resonance (NMR) scanning system, the NMR-MOUSE? (NMR-MOUSE (Nuclear Magnetic Resonance Mobile Universal Surface Explorer) is a registered trademark of RWTH Aachen University), for measuring porosity of geological drill core sections. The NMR-MOUSE? is used for transverse relaxation measurements on water-saturated core sections using a CPMG sequence with a short echo time. A regularized Laplace-transform analysis by the UPEN program yields the distribution of transverse relaxation times. The signal amplitudes and the distribution integrals correlate directly with the porosity of the cores, in spite of the influence of diffusion in the strong field gradient of the NMR-MOUSE?, which is discussed. The method is particularly attractive because it neither requires a volume calibration nor the samples to be machined to fit the coil, and because the device is mobile and particularly attractive for field use such as on logging platforms and research vessels.

Reactive flow and permeability prediction — numerical simulation of complex hydrogeothermal problems

Abstract Simulating complex flow situations in hydrogeothermal reservoirs requires coupling of flow, heat transfer, transport of dissolved species, and heterogeneous geochemistry. We present results of simulations for typical applications using the numerical simulator SHEMAT/Processing SHEMAT. Heat transfer is non-linear, since all thermal fluid and rock properties depend on temperature. Due to the coupling of fluid density with both temperature and concentrations of dissolved species, the model is well suited to simulate density-driven flow. Dissolution and precipitation of minerals are calculated with an improved version of the geochemical modelling code PHRQPITZ, which accurately calculates geochemical reactions in brines of low to high ionic strength and temperatures of 0–150°C. Changes in pore space structure and porosity are taken into account by updating permeability with respect to porosity changes due to precipitation and dissolution of minerals. This is based on a novel relationship between porosity and permeability, derived from a fractal model of the pore space structure and its changes due to chemical water — rock interaction. A selection of model studies performed with SHEMAT completes the review. Examples highlight both density-driven and reactive flow with permeability feedback. With respect to the former, the thermohaline free convection Elder’s problem, and density-driven free convection in a coastal aquifer with geothermal exploitation, are considered. Mineral redistribution and associated permeability change during a core flooding experiment; reaction front fingering in reservoir sandstone; and long-term changes in reservoir properties during the operation of a geothermal installation, are all considered in relation to reactive flow with permeability feedback.

Segmentation of thin section images for grain size analysis using region competition and edge-weighted region merging

Microscopic thin section images are a major source of information on physical properties, crystallization processes, and the evolution of rocks. Extracting the boundaries of grains is of special interest for estimating the volumetric structure of sandstone. To deal with large datasets and to relieve the geologist from a manual analysis of images, automated methods are needed for the segmentation task. This paper evaluates the region competition framework, which also includes region merging. The procedure minimizes an energy functional based on the Minimum Description Length (MDL) principle. To overcome some known drawbacks of current algorithms, we present an extension of MDL-based region merging by integrating edge information between adjacent regions. In addition, we introduce a modified implementation for region competition for overcoming computational complexities when dealing with multiple competing regions. Commonly used methods are based on solving differential equations for describing the movement of boundaries, whereas our approach implements a simple updating scheme. Furthermore, we propose intensity features for reducing the amount of data. They are derived by comparing theoretical values obtained from a model function describing the intensity inside uniaxial crystals with measured data. Error, standard deviation, and phase shift between the model and intensity measurements preserve sufficient information for a proper segmentation. Additionally, identified objects are classified into quartz grains, anhydrite, and reaction fringes by these features. This grouping is, in turn, used to improve the segmentation process further. We illustrate the benefits of this approach by four samples of microscopic thin sections and quantify them in a comparison of a segmentation result and a manually obtained one. HighlightsIntroduce additional gradient-based term for edge-weighted MDL-based region merging.Proposed term allows better balancing of the merging order regarding smaller and larger regions.Validity of our approach is tested on several thin section images.Validity of our approach is tested against a manually segmented image.

Modeling Chemical Brine-Rock Interaction in Geothermal Reservoirs

Application of the Debye-Huckel theory for chemical reaction modeling of geothermal brines does not yield sufficiently accurate results. Thus, for the development of a new chemical reaction module for the numerical simulation model SHEMAT (Clauser and Villinger, 1990), the Pitzer formalism (Pitzer, 1991) is used to calculate aqueous speciation and mineral solubilities. It is based on an extended code of PHRQPITZ (Plummer et al., 1988). Using temperature dependent Pitzer coefficients, the system Na-K-Mg-Ca-Ba-Sr-Si-H-Cl-SO4-OH-(HCO3-CO3-CO2)-H2O can be modeled with sufficient accuracy for temperatures from 0° to 150°C. The incorporated carbonic acid system (set in parentheses in the list above) is valid for temperatures from 0 to 90°C, only. Flow, heat transfer, species transport, and geochemical reactions are mutually coupled for modeling reactive flow. Changes in porosity and permeability influence the flow and transport properties of the reservoir. These changes are taken into account by a relation derived from a fractal model of the pore space structure (Pape et al., 1999).